A large variety of motorized systems for moving cargo are known. Motor driven rollers are typically employed in these systems. Cargo and passenger airplanes in particular often employ a series of motor driven power drive units (“PDU”s) to quickly and efficiently propel cargo containers and pallets, otherwise known as unit load devices (“ULD”s), within the aircraft cargo compartment. This configuration can allow for the transportation of cargo from the external loader to the interior of the airplane by one or more operators controlling the PDUs.
Cargo within an airplane cargo deck is typically supported by a system of freely rotating floor-mounted conveyance rollers. Sets or banks of PDUs can be simultaneously elevated from beneath the cargo deck to a level just above the conveyance rollers. Each PDU may be a separate electromechanical actuator which includes one or more rubber coated wheels or drive rollers. The drive rollers of the elevated PDUs contact and move cargo above the conveyance rollers in the commanded direction upon energization. The movement of cargo depends on the coefficient of friction between the PDU drive rollers and the bottom surface of the ULD, as well as the lifting force generated by the PDU lift mechanism. When the PDUs are deenergized, roller rotation ceases and the ULD stops moving.
Several sets of PDUs can be arranged along a common path of conveyance, and each set can be operated separately, thereby allowing for the transfer of multiple pieces of cargo. An operator supervising the transportation of cargo into the cargo deck area can guide cargo by means of a joystick and an on/off switch or similar controls.
PDUs can be damaged when they continue to operate beneath immobilized cargo, a condition known as scrubbing. Scrubbing occurs when cargo is too heavy or has come upon an obstruction such as a wall guide within the cargo compartment. Scrubbing can quickly wear away the rubber coating on the rollers (or the roller itself) necessitating their replacement and can result in damage to the PDU motor.
Cargo container stall sensors integrated within a PDU are used to sense a stalled container and to remove power to the PDU motor after a predetermined delay to avoid PDU damage. Some PDU control systems have a manual de-select switch for removing power to the PDUs when a stall condition is determined. Unfortunately, this de-select switch is often not used properly by operators, who are focused on loading cargo rather than protecting PDUs. Thus, damage to PDUs when scrubbing conditions occur is a common problem.
Known stall sensors include mechanisms for monitoring the temperature of the PDU motor, which is subject to measurement error, or require additional electromechanical mechanisms on the PDU, which are susceptible to wear and other maintenance issues.
Further, in the aircraft cargo area, it is important to keep track of the location of the ULDs. The most common method of keeping track of these ULDs, while they are in the cargo area, is by detecting them as they pass over a ULD sensor which is located on the floor of the cargo compartment. One known sensing method is the use of infrared (IR) light to determine the presence of the ULD. For instance, U.S. Pat. No. 5,661,384 discloses a PDU having an IR sensor to detect the presence of cargo directly above a corresponding PDU. U.S. Pat. No. 7,014,038 also discloses employing an IR or other sensor data to detect cargo. Such systems typically employ a digital sensor which only allows two states (i.e., “ULD present” or “ULD not present”). While prior art PDUs are configured to use IR information to detect an ULD, they are not configured to detect whether the ULD is moving.